Compatibility of cardiac pacemakers, implantable defibrillators and other types of active implantable medical devices with magnetic resonance imaging (MRI) and other types of hospital diagnostic equipment has become a major issue. If one goes to the websites of the major cardiac pacemaker manufacturers in the United States, which include St. Jude Medical, Medtronic and Boston Scientific CRM (formerly Guidant), one will see that the use of MRI is generally contra-indicated with pacemakers and implantable defibrillators. A similar contra-indication is found in the manuals of MRI equipment manufacturers such as Siemens, GE, and Phillips. See also “Safety Aspects of Cardiac Pacemakers in Magnetic Resonance Imaging”, a dissertation submitted to the Swiss Federal Institute of Technology Zurich presented by Roger Christoph Lüchinger. “Dielectric Properties of Biological Tissues: I. Literature Survey”, by C. Gabriel, S. Gabriel and E. Cortout; “Dielectric Properties of Biological Tissues: II. Measurements and the Frequency Range 0 Hz to 20 GHz”, by S. Gabriel, R. W. Lau and C. Gabriel; “Dielectric Properties of Biological Tissues: Ill. Parametric Models for the Dielectric Spectrum of Tissues”, by S. Gabriel, R. W. Lau and C. Gabriel; and “Advanced Engineering Electromagnetics, C. A. Balanis, Wiley, 1989, all of which are incorporated herein by reference.
However, an extensive review of the literature indicates that MRI is indeed often used with pacemaker patients in spite of the contra indications. The safety and feasibility of MRI in patients with cardiac pacemakers is an issue of gaining significance. The effects of MRI on patients' pacemaker systems have only been analyzed retrospectively in some case reports. MRI is one of medicine's most valuable diagnostic tools. MRI is, of course, extensively used for imaging, but is also increasingly used for real-time procedures such as interventional medicine (surgery). In addition, MRI is used in real time to guide ablation catheters, neurostimulator tips, deep brain probes and the like. An absolute contra-indication for pacemaker patients means that pacemaker and ICD wearers are excluded from MRI. This is particularly true of scans of the thorax and abdominal areas. However, because of MRI's incredible value as a diagnostic tool for imaging organs and other body tissues, many physicians simply take the risk and go ahead and perform MRI on a pacemaker patient. The literature indicates a number of precautions that physicians should take in this case, including limiting the applied power of the MRI in terms of the specific absorption rate—SAR programming the pacemaker to fixed or asynchronous pacing mode, having emergency personnel and resuscitation equipment standing by (known as “Level II” protocol), and careful reprogramming and evaluation of the pacemaker and patient after the procedure is complete. There have been reports of latent problems with cardiac pacemakers after an MRI procedure occurring many days later (such as increase in or loss of pacing pulse capture).
There are three types of electromagnetic fields used in an MRI unit. The first type is the main static magnetic field designated B0 which is used to align protons in body tissue. The field strength varies from 0.5 to 3.0 Tesla in most of the currently available MRI units in routine clinical use.
The second type of field produced by magnetic resonance imaging equipment is the pulsed RF field which is generated by the body coil or head coil, also referred to as B1. This is used to change the energy state of the protons and illicit MRI signals from tissue. The RF field is homogeneous in the central region and has two main components: (1) the magnetic field is circularly polarized in the actual plane; and (2) the electric field is related to the magnetic field by Maxwell's equations. The frequency of the RF pulsed varies with the field strength of the main static field, as expressed in the Lamour Equation: RF PULSED FREQUENCY (in MHz)=(MRI CONSTANT) (STATIC FIELD STRENGTH (T).
The third type of electromagnetic field is the time-varying magnetic gradient field designated Gx, y, z which is used for spatial localization. The gradient field changes its strength along different orientations and operating frequencies on the order of 1 to 2.2 kHz.
At the pulsed RF frequencies of interest in MRI, RF energy can be absorbed and converted to heat. The power deposited by RF pulses during MRI is complex and is dependent upon the power, duration and shape of the RF pulse, the relative long term time averages of the pulses, the transmitted frequency, the number of RF pulses applied per unit time, and the type of configuration of the RF transmitter coil used. Specific absorption rate (SAR) is a measure of how much energy is induced into body tissues. The amount of heating also depends upon the volume of the various tissue (i.e. muscle, fat, etc.) imaged, the electrical resistivity of tissue and the configuration of the anatomical region imaged. There are also a number of other variables that depend on the placement in the human body of the AIMD and its associated lead wire(s). For example, it will make a difference how much current is induced into a pacemaker lead wire system as to whether it is a left or right pectoral implant. In addition, the routing of the lead and the lead length are also very critical as to the amount of induced current and heating that would occur. Location within the MRI bore is also important since the electric fields required to generate the RF increase exponentially as the patient is moved away from MRI bore center-line (ISO center). The cause of heating in an MRI environment is twofold: (a) RF field coupling to the lead can occur which induces significant local heating; and (b) currents induced during the RF transmission can flow into body tissue and cause local Ohm's Law heating next to the distal TIP electrode of the implanted lead. The RF field in an MRI scanner can produce enough energy to induce lead wire currents sufficient to destroy some of the adjacent myocardial tissue. Tissue ablation has also been observed. The effects of this heating are not readily detectable by monitoring during the MRI. Indications that heating has occurred would include an increase in pacing threshold, venous ablation, Larynx ablation, myocardial perforation and lead penetration, or even arrhythmias caused by scar tissue. Such long term heating effects of MRI have not been well studied yet.
It has been observed that the RF field may induce undesirable fast cardiac pacing (QRS complex) rates. There are various mechanisms which have been proposed to explain rapid pacing: direct tissue stimulation, interference with pacemaker electronics or pacemaker reprogramming (or reset). In all of these cases, it would be desirable to raise the lead system impedance (to reduce RF current), make the feedthrough capacitor more effective and provide a very high degree of protection to AIMD electronics. This will make alterations in pacemaker pacing rate and/or pacemaker reprogramming much more unlikely.
As one can see, many of the undesirable effects in an implanted lead wire system from MRI and other medical diagnostic procedures are related to undesirable induced currents in the lead wire system. This can lead to overheating either in the lead wire or at the tissue interface at the distal Tip electrode.
Bandstop filters employing a capacitor and an inductor tank circuit can be used to enhance the MRI compatibility of active implantable medical device implanted leads. These are described in U.S. Pat. No. 7,363,090, the contents of which are incorporated herein. The bandstop filters of the '090 patent are designed to be resonant at a selected MRI RF pulse center frequency. The bandstop filters are effective over a range of selected RF pulsed frequencies, which means that they have a broad enough bandwidth to accommodate most manufacturers of 1.5 Tesla rated scanners. The bandstop filters could also be designed for 3 Tesla, 5 Tesla, or other specified scanners. However, it would be exceedingly dangerous to assume that if an implanted lead bandstop filter was rated for 3 Tesla that one could safely perform 1.5 Tesla scans. This is because the RF pulsed frequency between a 3 Tesla and a 1.5 Tesla scanner is very different (approximately 128 MHz for a 3 Tesla and approximately 64 MHz for a 1.5 Tesla). A bandstop filter designed to be resonant for 3 Tesla systems and present a very high impedance, would undesirably present a very low impedance at 1.5 Tesla.
Accordingly, there is a need for a rapid and convenient means of identification of the MRI compatibility of an implanted lead and, in particular, its associated bandstop filter. Moreover, there is a need for a means of rapidly identifying the type of bandstop filter that is associated with an implanted lead and its MRI compatibility. The present invention fulfills these needs and provides other related advantages.